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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"><HTML><HEAD><TITLE>A Detailed Introduction to I/O and I/O Redirection</TITLE><METANAME="GENERATOR"CONTENT="Modular DocBook HTML Stylesheet Version 1.76b+"><LINKREL="HOME"TITLE="Advanced Bash-Scripting Guide"HREF="index.html"><LINKREL="PREVIOUS"TITLE="Exit Codes With Special Meanings"HREF="exitcodes.html"><LINKREL="NEXT"TITLE="Command-Line Options"HREF="command-line-options.html"><METAHTTP-EQUIV="Content-Style-Type"CONTENT="text/css"><LINKREL="stylesheet"HREF="common/kde-common.css"TYPE="text/css"><METAHTTP-EQUIV="Content-Type"CONTENT="text/html; charset=iso-8859-1"><METAHTTP-EQUIV="Content-Language"CONTENT="en"><LINKREL="stylesheet"HREF="common/kde-localised.css"TYPE="text/css"TITLE="KDE-English"><LINKREL="stylesheet"HREF="common/kde-default.css"TYPE="text/css"TITLE="KDE-Default"></HEAD><BODYCLASS="APPENDIX"BGCOLOR="#FFFFFF"TEXT="#000000"LINK="#AA0000"VLINK="#AA0055"ALINK="#AA0000"STYLE="font-family: sans-serif;"><DIVCLASS="NAVHEADER"><TABLESUMMARY="Header navigation table"WIDTH="100%"BORDER="0"CELLPADDING="0"CELLSPACING="0"><TR><THCOLSPAN="3"ALIGN="center">Advanced Bash-Scripting Guide: An in-depth exploration of the art of shell scripting</TH></TR><TR><TDWIDTH="10%"ALIGN="left"VALIGN="bottom"><AHREF="exitcodes.html"ACCESSKEY="P">Prev</A></TD><TDWIDTH="80%"ALIGN="center"VALIGN="bottom"></TD><TDWIDTH="10%"ALIGN="right"VALIGN="bottom"><AHREF="command-line-options.html"ACCESSKEY="N">Next</A></TD></TR></TABLE><HRALIGN="LEFT"WIDTH="100%"></DIV><DIVCLASS="APPENDIX"><H1><ANAME="IOREDIRINTRO"></A>Appendix E. A Detailed Introduction to I/O and I/O Redirection</H1><P><SPANCLASS="emphasis"><ICLASS="EMPHASIS">written by St閜hane Chazelas, and revised by the document author</I></SPAN></P><P><ANAME="STDINOUTDEF"></A></P><P>A command expects the first three <AHREF="io-redirection.html#FDREF">file descriptors</A> to be available. The first, <ICLASS="FIRSTTERM">fd 0</I> (standard input, <TTCLASS="FILENAME">stdin</TT>), is for reading. The other two (<ICLASS="FIRSTTERM">fd 1</I>, <TTCLASS="FILENAME">stdout</TT> and <ICLASS="FIRSTTERM">fd 2</I>, <TTCLASS="FILENAME">stderr</TT>) are for writing.</P><P>There is a <TTCLASS="FILENAME">stdin</TT>, <TTCLASS="FILENAME">stdout</TT>, and a <TTCLASS="FILENAME">stderr</TT> associated with each command. <TTCLASS="USERINPUT"><B>ls 2>&1</B></TT> means temporarily connecting the <TTCLASS="FILENAME">stderr</TT> of the <BCLASS="COMMAND">ls</B> command to the same <SPANCLASS="QUOTE">"resource"</SPAN> as the shell's <TTCLASS="FILENAME">stdout</TT>.</P><P>By convention, a command reads its input from fd 0 (<TTCLASS="FILENAME">stdin</TT>), prints normal output to fd 1 (<TTCLASS="FILENAME">stdout</TT>), and error ouput to fd 2 (<TTCLASS="FILENAME">stderr</TT>). If one of those three fd's is not open, you may encounter problems:</P><TABLEBORDER="0"BGCOLOR="#E0E0E0"WIDTH="100%"><TR><TD><PRECLASS="SCREEN"> <TTCLASS="PROMPT">bash$ </TT><TTCLASS="USERINPUT"><B>cat /etc/passwd >&-</B></TT> <TTCLASS="COMPUTEROUTPUT">cat: standard output: Bad file descriptor</TT> </PRE></TD></TR></TABLE><P>For example, when <BCLASS="COMMAND">xterm</B> runs, it first initializes itself. Before running the user's shell, <BCLASS="COMMAND">xterm</B> opens the terminal device (/dev/pts/<n> or something similar) three times.</P><P>At this point, Bash inherits these three file descriptors, and each command (child process) run by Bash inherits them in turn, except when you redirect the command. <AHREF="io-redirection.html#IOREDIRREF">Redirection</A> means reassigning one of the file descriptors to another file (or a pipe, or anything permissible). File descriptors may be reassigned locally (for a command, a command group, a <AHREF="subshells.html#SUBSHELLSREF">subshell</A>, a <AHREF="redircb.html#REDIRREF">while or if or case or for loop</A>...), or globally, for the remainder of the shell (using <AHREF="internal.html#EXECREF">exec</A>).</P><P><TTCLASS="USERINPUT"><B>ls > /dev/null</B></TT> means running <BCLASS="COMMAND">ls</B> with its fd 1 connected to <TTCLASS="FILENAME">/dev/null</TT>.</P><P> <TABLEBORDER="0"BGCOLOR="#E0E0E0"WIDTH="100%"><TR><TD><PRECLASS="SCREEN"> <TTCLASS="PROMPT">bash$ </TT><TTCLASS="USERINPUT"><B>lsof -a -p $$ -d0,1,2</B></TT> <TTCLASS="COMPUTEROUTPUT">COMMAND PID USER FD TYPE DEVICE SIZE NODE NAME bash 363 bozo 0u CHR 136,1 3 /dev/pts/1 bash 363 bozo 1u CHR 136,1 3 /dev/pts/1 bash 363 bozo 2u CHR 136,1 3 /dev/pts/1</TT> <TTCLASS="PROMPT">bash$ </TT><TTCLASS="USERINPUT"><B>exec 2> /dev/null</B></TT> <TTCLASS="PROMPT">bash$ </TT><TTCLASS="USERINPUT"><B>lsof -a -p $$ -d0,1,2</B></TT> <TTCLASS="COMPUTEROUTPUT">COMMAND PID USER FD TYPE DEVICE SIZE NODE NAME bash 371 bozo 0u CHR 136,1 3 /dev/pts/1 bash 371 bozo 1u CHR 136,1 3 /dev/pts/1 bash 371 bozo 2w CHR 1,3 120 /dev/null</TT> <TTCLASS="PROMPT">bash$ </TT><TTCLASS="USERINPUT"><B>bash -c 'lsof -a -p $$ -d0,1,2' | cat</B></TT> <TTCLASS="COMPUTEROUTPUT">COMMAND PID USER FD TYPE DEVICE SIZE NODE NAME lsof 379 root 0u CHR 136,1 3 /dev/pts/1 lsof 379 root 1w FIFO 0,0 7118 pipe lsof 379 root 2u CHR 136,1 3 /dev/pts/1</TT> <TTCLASS="PROMPT">bash$ </TT><TTCLASS="USERINPUT"><B>echo "$(bash -c 'lsof -a -p $$ -d0,1,2' 2>&1)"</B></TT> <TTCLASS="COMPUTEROUTPUT">COMMAND PID USER FD TYPE DEVICE SIZE NODE NAME lsof 426 root 0u CHR 136,1 3 /dev/pts/1 lsof 426 root 1w FIFO 0,0 7520 pipe lsof 426 root 2w FIFO 0,0 7520 pipe</TT></PRE></TD></TR></TABLE></P><P>This works for different types of redirection.</P><P><TTCLASS="USERINPUT"><B>Exercise:</B></TT> Analyze the following script.<TABLEBORDER="0"BGCOLOR="#E0E0E0"WIDTH="100%"><TR><TD><PRECLASS="PROGRAMLISTING"> 1 #! /usr/bin/env bash 2 3 mkfifo /tmp/fifo1 /tmp/fifo2 4 while read a; do echo "FIFO1: $a"; done < /tmp/fifo1 & exec 7> /tmp/fifo1 5 exec 8> >(while read a; do echo "FD8: $a, to fd7"; done >&7) 6 7 exec 3>&1 8 ( 9 ( 10 ( 11 while read a; do echo "FIFO2: $a"; done < /tmp/fifo2 | tee /dev/stderr \ 12 | tee /dev/fd/4 | tee /dev/fd/5 | tee /dev/fd/6 >&7 & exec 3> /tmp/fifo2 13 14 echo 1st, to stdout 15 sleep 1 16 echo 2nd, to stderr >&2 17 sleep 1 18 echo 3rd, to fd 3 >&3 19 sleep 1 20 echo 4th, to fd 4 >&4 21 sleep 1 22 echo 5th, to fd 5 >&5 23 sleep 1 24 echo 6th, through a pipe | sed 's/.*/PIPE: &, to fd 5/' >&5 25 sleep 1 26 echo 7th, to fd 6 >&6 27 sleep 1 28 echo 8th, to fd 7 >&7 29 sleep 1 30 echo 9th, to fd 8 >&8 31 32 ) 4>&1 >&3 3>&- | while read a; do echo "FD4: $a"; done 1>&3 5>&- 6>&- 33 ) 5>&1 >&3 | while read a; do echo "FD5: $a"; done 1>&3 6>&- 34 ) 6>&1 >&3 | while read a; do echo "FD6: $a"; done 3>&- 35 36 rm -f /tmp/fifo1 /tmp/fifo2 37 38 39 # For each command and subshell, figure out which fd points to what. 40 # Good luck! 41 42 exit 0</PRE></TD></TR></TABLE> </P></DIV><DIVCLASS="NAVFOOTER"><HRALIGN="LEFT"WIDTH="100%"><TABLESUMMARY="Footer navigation table"WIDTH="100%"BORDER="0"CELLPADDING="0"CELLSPACING="0"><TR><TDWIDTH="33%"ALIGN="left"VALIGN="top"><AHREF="exitcodes.html"ACCESSKEY="P">Prev</A></TD><TDWIDTH="34%"ALIGN="center"VALIGN="top"><AHREF="index.html"ACCESSKEY="H">Home</A></TD><TDWIDTH="33%"ALIGN="right"VALIGN="top"><AHREF="command-line-options.html"ACCESSKEY="N">Next</A></TD></TR><TR><TDWIDTH="33%"ALIGN="left"VALIGN="top">Exit Codes With Special Meanings</TD><TDWIDTH="34%"ALIGN="center"VALIGN="top"> </TD><TDWIDTH="33%"ALIGN="right"VALIGN="top">Command-Line Options</TD></TR></TABLE></DIV></BODY></HTML>⌨️ 快捷键说明
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